Halo 4's Master Chief "changed"

That means psychological changes, mind. Not just the physical loss of his Spartan codpiece. Although that has to affect a guy.

"There's some story behind his new look," studio boss Frank O'Connor told OXM. "There are specific reasons why he looks the way he does and why some of his behaviours have changed. We'll see in good time."

Some time has passed since the events of Halo 3, when the Chief finished the fight. Halo 4 opens with him floating through space towards an alien planet. "A lot of things have changed in that intervening period," O'Connor teased.